You have the right to file an answer if your spouse filed for divorce and you were served with divorce papers. According to TexasLawHelp.org, an answer is "a legal form you (the respondent) file with the court to protect your right to have a say in the divorce."
If you do not file an answer, the divorce can go ahead without you. Your spouse will get a default judgment. You will not have any input about what happens to your property, your debts, and possibly issues involving your children.
If the divorce is contested, you also have the opportunity to file a counter-petition. Filing a counter-petition lets you tell the judge what you would like to happen instead.
An answer to a divorce petition must be filed within a certain timeframe. If you are served in person or by certified mail, you need to submit your answer by 10 am on the Monday after 20 days have gone by since you were served with papers. TexasLawHelp.org explains how to find your answer's due date:
To determine the deadline, find the day you were served with divorce papers on a calendar, count out 20 more days (including weekends and holidays) then go to the next Monday. You must file an answer with the court on or before this date at 10:00 a.m. If you don’t, your spouse can finish the divorce without you (as long as any other applicable waiting periods have passed). Note: If the courts are closed on the day your answer is due, then your answer is due the next day the courts are open.
If you do not want to be formally served with divorce papers, you can file a waiver of service. This means that your spouse doesn't need to serve you with papers as described on the "Serving Divorce Papers" page of this guide. You will still have the right to be involved in the proceedings. It does not waive any of your rights except the formal notice of the suit.
If you're not sure whether you should file a waiver of service or an answer, please check with an attorney about what would be best for you.